Hello,

I want to build an ultrasonic radar circuit (ultrasonic motion detector). The one I attached is widespread on the websites that provide schematics..
I know about someone who built it and found some problems with it, and he fixed them. He said that it worked well till a distance of about 7m. I'm trying to find the problem of the circuit before I make the PCB (which is provided) and so I can edit it before etching.

My view:
TR3 emitter should be downward (this may be my friends fault).
I want to ask about IC1's inputs, should they be reversed or they are correctly connected?

I may use LM358 instead of 741, but this will make me design another PCB. If this increases the range much more then I will.

I do understand how the circuit works in general.

The parts list and information are provided at the end of this page:
http://english.cxem.net/guard/guard7.php

Any help finding the problem/problems? any suggestions?

Regards,
Hazim

http://english.cxem.net/guard/guard7.php

 

C1 on the schematic shorts the receiving transducer (the negative wire of C1 should connect to ground) but the pcb is correct.
C3 was used with very old opamps that are obsolete today so C3 is not needed.

IC1's inputs are correct.

The 741 opamp does not have enough gain at 40kHz and the LM358 dual opamp is worse.
Use TL071 single wideband opamps or a TL072 dual wideband opamps instead.

 

Right, it's clear from the datasheets that TL072 is much better than the other two op-amps.. I see that point 3, at the receiving transducer should be connected to ground, it's like that in the pcb diagram... If the pcb is correct then the only real problem is with C1 as you said.